A display device of a head mounted type by which a viewer views a display unit such as an LCD (Liquid Crystal Display) through an optical system has been conventionally proposed (for example, JP2011-509417A). In such a display device of a head mounted type, a light guide plate is located at a position opposed to the display unit. Thus, image light displayed by the display unit is guided by the light guide plate up to a position corresponding to eyes of the viewer, and the light is reflected by a reflection layer toward the viewer. In this display device, since a field of view of the viewer is blocked by a displayed image, a magic mirror is used in the reflection layer, for example, to obtain a see-through condition in which the viewer sees the image and external light that are overlapped. However, in this case, when a transmission factor of the magic mirror is improved to excessively lower a reflection factor, an image getting through to the viewer may become unclear. On the other hand, when the reflection factor is improved to excessively lower the transmission factor, the external light (external image) may become unclear.